I think you actually understand the philosophical basis of property and why it doesn't apply to ideas better than you're letting on here, but I'll answer this anyway.
Ownership of physical property is natural because physical property is inherently rivalrous and excludable — if you take my hammer, I can't use it, and thus I need to keep my hammer in my shed if I want to have access to it when I need it. Ownership makes natural sense there. It's essential to security.
Ideas are not the same way. Any number of people can think an idea, and in fact an idea becomes just as much a part of everyone who thinks it as the first person who thought it, and you literally can't take an idea back once you have shared it. Ideas are not just easy to copy, they aren't even entities that need copying. The mind is constantly taking in and assimilating ideas and creating its own.
I'm not actually against copyright. I think there is a sound moral basis for it, in that otherwise an increasing number of people wouldn't be able to enjoy the fruits of their labor. I just don't think there is a moral justification for the tortured concept of eternal copyright. Eventually, any successful idea will become so diffused throughout society that it's just an utter farce to pretend it is not part of the commons.
If nothing else, infinitely decreasing the number of ideas that may be legally had is so harmful to society that it outweighs any good that could possibly come from it. In other words, even if we say "Sure, let's pretend treating ideas as property is a natural fit," the Lockean Proviso warns us against allowing them to remain property forever.